CHAPTER XXXII 



THE PRODUCTION AND REGULATION OF THE HEAT OF 



THE BODY 



Cold- and Warm-Blooded Animals. All animals, so long as 

 they are alive, are the seat of chemical changes by which heat is 

 liberated; hence all tend to be somewhat warmer than their or- 

 dinary surroundings, though the difference may not be noticeable 

 unless the heat production is considerable. A frog or a fish is a 

 little hotter than the air or water in which it lives, but not much; 

 the little heat that it produces is lost, by radiation or conduction, 

 almost at once. Hence such animals have no proper temperature 

 of their own; on a warm day they are warm, on a cold day cold, 

 and are accordingly known as changeable-temper -atured (poikilo- 

 thermous) or, in ordinary language, "cold-blooded" animals. 

 Man and other mammals, as well as birds, on the contrary, are 

 the seat of very active chemical changes by which much heat is 

 produced, and so maintain a tolerably uniform temperature of 

 their own, much as a fire does whether it be burning in a warm or 

 a cold room ; the heat production during any given time balancing 

 the loss, a normal body temperature is maintained, and usually 

 one considerably higher than that of the medium in which they 

 live; such animals are commonly named "warm-blooded." This 

 name, ' however, does not properly express the facts; a lizard 

 basking in the sun on a warm summer's day may be quite as hot 

 as a man usually is; but on the cold day the lizard becomes cold, 

 while the average temperature of the healthy Human Body is, 

 within a degree, the same in winter or summer; within the arctic 

 circle or on the equator. Hence it is better to call such animals 

 " homothermous " or of uniform temperature. 



Moderate warmth accelerates protoplasmic activity; compare 

 a frog dormant in the winter with the same animal active in the 

 warm months : what is true of the whole frog is true of each of its 

 living cells. Its muscles contract more rapidly when warmed, 

 and the white corpuscles of its blood when heated up to the tern- 



639 



